This invention relates generally to MISFET (MOSFET) transistors, and more particularly the invention relates to a MOSFET structure having reduced gate to drain feedback capacitance.
The MISFET transistor, more commonly known as a MOSFET, includes a source and drain region separated by a channel region with the conductance of the channel region controlled by a voltage bias applied to a gate electrode over the channel. Ohmic contacts are made to each of the source, drain, and gate of a transistor in circuit applications.
Discrete MOSFET devices are well suited for RF/microwave power applications. The gate to drain feedback capacitance (C.sub.gd or C.sub.rss) of any MOSFET transistor device must be minimized in order to maximize RF signal gain and minimize signal distortion. The gate to drain feedback capacitance is critical since it is effectively multiplied by the voltage gain of the device, or C.sub.effective =C.sub.rss (1+gmR.sub.1) where gm is the transconductance and R.sub.1, is the load impedance.
Attempts to minimize the gate to drain feedback capacitance includes the use of a Faraday shield made of metal or polysilicon which is formed over the gate structure and typically is connected to the source region. See for example, Technical Digest IEDM Conference 1996, pgs. 87-90 which describe a 2 GHz RF LDMOS transistor and pgs. 91-94 which describe a 1 GHz RF LDMOS transistor. FIG. 1A shows a plurality of RF/microwave power MOSFET transistors in a semiconductor body 12 with bonding pads provided between adjacent transistors with the bonding pads having capacitive coupling as illustrated in FIG. 1B. In these devices a plurality of MOSFET transistors are linearly arranged in a semiconductor body 12 with shared gate bond pads 16 and drain bond pads 15 provided between adjacent transistors on a major surface of a semiconductor body. All of the bond pads are generally linearly arranged, and bond wires 15, 17 contact the respective drain pads and gate pads, as illustrated in the plan view of FIG. 1 of the drawing. The close proximity of the bond pads and the bond wires provides a coupling capacitance between the drain and gate which is not negligible. For example, the Crss component from gate to drain bond wires is estimated at greater than 10% of the total gate-drain capacitance for a drain to source voltage greater than 10 volts.
The present invention is directed to an integrated MOSFET transistor device having reduced gate to drain feedback capacitance.